Childhood obesity is one of the major health threats for modern American society. A major concern regarding childhood obesity is that obese children face an increased risk of diabetes mellitus, hypertension, stroke and coronary artery diseases. Obesity is a particularly challenging medical condition to treat because of its multi-factorial etiology and existing therapeutic limitations. The melanocortin-4 receptor (MC4R) has been identified to play a key role in the regulation of food intake and body weight. Our long term goal is to elucidate the molecular basis of hMC4R responsible for ligand binding and signaling as a necessary prerequisite to the development of selective MC4R nonpeptide agonist for therapeutic treatment of obesity. We hypothesize that unique amino acid residues in the transmembrane domains of hMC4R are involved in the selective MC4R agonist binding. To test this hypothesis, we will utilize 1) novel photoaffinity labeling of selective agonist to examine the direct interaction between the ligand and MC4R, and 2) identify the direct interactions between selective agonist and MC4R using reciprocal ligand and receptor residue exchange. Our proposed studies will provide the finest level of molecular detail for ligand binding, receptor signaling and will provide valuable information for designing selective MC4R agonists that may be used in the treatment of human obesity. Obesity is one of the major health threats facing modern American society. This R03 application is to explore the molecular basis of melanocorin 4 receptor responsible for agonist activity which can be used to develop new therapuetic approach for obesity.